In a urea spray SCR control, control is performed to attain an appropriate amount of urea spray in accordance with an engine-out NOx discharge amount detected by an NOx sensor (Patent Document 1). When the amount of urea spray is too large, ammonia which is a toxic substance is discharged, and when the amount of urea spray is too small, the NOx purification rate is reduced, which affects the exhaust gas performance.
The engine-out the amount of NOx can be detected by an NOx sensor with a high degree of accuracy, which is therefore generally employed, but it takes a certain amount of time for the NOx sensor to become ready to sense the amount of NOx after the activation of the sensor. Due to the structure of the NOx sensor, it is necessary to keep a sensing unit (hereinafter referred to as an element unit) at a high temperature region (e.g., 800° C.); therefore, heating control is performed. Ceramics are used for the element unit; therefore, the element unit is damaged if waterdrops and the like attach to the element when it is in a heated state. Accordingly, when the NOx sensor is used, pre-heating is performed until there is no longer any moisture included in the moisture content included in the exhaust gas immediately after the engine is started (before warm up) and there is no longer any waterdrops generated by condensation and the like in the discharge pipe and by the NOx sensor itself. This pre-heating is to evaporate waterdrops generated within the sensor due to condensation by heating the element at such a temperature that the element is not damaged even when the waterdrops attach to the element (for example, 100 degrees). As described above, the heating control in the high temperature region is performed after it is made sure that there is no longer any chance of waterdrops attached thereto in the pre-heating (hereinafter this control will be referred to as pre-heating control), and thereafter the amount of NOx is sensed (Patent Document 2, 3).
During the time in which the pre-heating control is performed, the amount of NOx cannot be detected, and therefore, in the urea spray control during that period, the amount of injection is determined using NOx model map (the amount of NOx map for each of the engine rotation and the amount of injection commanded), instead of the sensor.